Generally, a tool is mounted on a rotational spindle of a machine tool such as a machining center or a milling machine through a chuck device. An example of the chuck device is disclosed in Patent Document 1, which is shown in FIG. 6. A chuck device 101 shown in FIG. 6 including a chuck body 102, a collet 103 and a nut 104 is well known. The nut 104 is configured to be screwed to the chuck body 102 for fastening the collet 103 to hold a tool T.
Since the nut 104 serves to fasten the collet 103 to reduce the diameter of the collet 103, an arrangement in which a force is exerted radially inward is required. The invention disclosed in Patent Document 1 was made to address such a subject matter, in which a tapered surface 103a formed in the collet 103 and a tapered surface 104a formed in the nut 104 are brought into contact with each other when the nut is screwed to the chuck body. More particularly, the collet 103 and the nut 104 are brought into contact with each other at the tapered surfaces, thereby to press the collet 103 axially and fasten the collet 103 radially inward.